This invention relates to "short" belt magnetic separators having a pulley head with magnets located therein that do not rotate with the pulley head, and particularly to a process for increasing the productivity of such separators.
Two basic types of endless belt magnetic separators are known in the art. In one type, called disc-type separators, the pulley head has magnets that rotate with the head. Previous testing of this type of separator has demonstrated that a capacity of approximately 30 to 35 long tons per hour per foot of magnet width (ltphpfmw) can be achieved with minus 1-inch magnetic taconite ore from USS's Minntac mine near Mountain Iron, Minn. With this type of magnetic separator, the strongly magnetic particles form magnetic floccules as they are "grabbed and held" until they are carried out of the magnetic field by the movement of the belt. To minimize the entrapment of non-magnetic particles within magnetic floccules, the layer of particles must be thin and, therefore, the capacity is low. In the other, "short" belt separator, the magnets are stationary during separator operation. I refer to this type as an "ore-agitating" type of separator because the magnetic particles (which are little magnets themselves) reorient and flip 180.degree. as they pass by the fixed internal magnets of alternating polarity. The magnets in this latter type of separator may be adjustable to various positions within the head but do not rotate with it. The term, "short" belt, arises from the restrictions on belt length due to the type of bearings required for the pulley of this type separator. The productivity or throughput of material on short belt separators has been somewhat limited due to the manner in which such separators are conventionally used. Under the theory of operation used in the past, it was thought to be necessary to maintain the magnetic ore particles in the magnetic field on the belt for at least a portion of the time in which the belt passes over the pulley head. Thus, conventional practice has been to operate the belt at a sufficiently slow speed so that the magnetic particles are not released from the magnetic field until they are turned downwardly in a shorter trajectory than the non-magnetic gangue particles to assure effective separation by splitters or baffles located outwardly of and beneath the upper level of the pulley head. Manufacturers of the short-belt, "ore-agitating" type of head pulley show in their catalogs a capacity on the order of 9 to 21 ltphpfmw at belt speeds of 250 to 490 feet per minute (fpm) for magnetic iron ores similar to the taconite ore mentioned above. Conventional magnetic head pulley designs have typically 5 to 9 pole pieces of alternating polarity magnets in a magnetic arc of about 120 to 180 degrees. The interpole design has (in addition) small bucking magnets between each pole piece. When a typical short-belt, dry magnetic head pulley separator is operated at a low speed, i.e. 250 to 400 fpm for a 48-inch diameter drum, none of the particles (magnetic or non-magnetic) leave the belt adjacent to the drum surface until they have undergone significant agitation by passing over several magnet poles of alternating polarity. At these conditions, the magnetic particles form magnetic floccules that entrap non-magnetic particles. In addition, the band of ore particles thrown from the belt is narrow and the exact splitter-position between magnetics and non-magnetics is very critical and gives variable results when the belt speed is low.
It is a primary object of this invention to increase the rate of productivity and separation efficiency of short-belt type magnetic separators having a pulley head with magnets located therein that do not rotate with the pulley head.